Problem with dissolved air flotation

Our plant has a new DAF prototype to treat surface water with FeCl3 as the coagulant. The flow rate for the raw water is 105 USG/min, and the pressurized water flow is 15 USG/min under 58 psi of pressure. The characteristics of the raw water are very low turbidity (less than 1 NTU), an apparent color of 40 to 100 Hazen, depending on the time of year, constant pH of 5.6, and a temperature that varies from 19 °C/66 °F in summer to occasionally below 1 °C/34 °F in winter. The raw water gets a dose of lime slurry to raise the pH before injection of FeCl3, which causes the pH to drop, so as to maintain a coagulation pH of 5.4.

Last summer, strong precipitation lead in an increase of the colour of the raw water, causing big disruptions in the water treatment process. It proved impossible to either increase or decrease the FeCl3 treatment without seeing coloured water saturated with floc, even if the white water stream was doing its job. We found that by lowering the coagulation pH to 4 while maintaining the rate of FeCl3, we were able to achieve satisfactory results.

Currently, raw water temperature has dropped precipitously to 1.9 °C/35.4 °F. It has been impossible to treat it with a coagulation pH of 4. Neither could we increase or decrease treatment without getting disappointing results in the floated water. So, we did some testing and it turns out that by increasing the coagulation pH to 4.4, we could ensure proper water, but we still don’t have an adequate grasp of the situation.

Can you help us figure out what the ratios are for DAF? We have a Hazen value of 70 and turbidity of 0.9 NTU at the raw water. We are dosing it with 55 ppm of FeCl3 with a coagulation pH of 4.4 and polymer at 0.3 g/m3. Which are the parameters we need to focus on first in order to improve our treatment process? Many thanks for any advice you can provide.

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In your DAF prototype (separator via flotation), how is the sludge that is produced extracted? What volume of sludge is produced/1000 USG treated?

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The sludge is extracted using hydraulics, i.e. closing off the float water valve causes a rise in the DAF tank level, and the sludge spills over. We treat 105 USG/min and program a sludge extraction every 5 h for a discharged volume of about 790 USG.